Added support for ambisonics projection using mapping 3 (disabled by default)

Fixes to the CELT PLC

Additionally, as a way to test the upcoming update to opus-tools, we’re providing Windows binaries
built with 1.3-beta. These binaries are based on libopusenc, which means opusenc is finally able
to make use of the Opus delayed-decision feature to make better speech/music transitions.